The invention relates to an improved process for producing ordered polymer foams from emulsifier foam precursors by microfluidic processes and to correspondingly produced polymer foams and to the use thereof.
Polymer foams have various uses, for example for thermal insulation, for mechanical damping, for sound absorption, as packaging materials or, for example, as water-absorbing, crosslinked polymers as superabsorbents. WO 97/17397 discloses water-absorbing crosslinked polymer foams which are obtainable by foaming a polymerizable mixture which comprises, inter alia, monoethylenically unsaturated monomers, the foaming being effected by dispersing fine bubbles of an inert gas and polymerizing the foamed mixture to form a hydrogel foam. The foam is produced separately from the polymerization, and the production can be carried out, for example, in industrial apparatus which is known for the production of urea-formaldehyde foams or, in a simple case, in a conventional food processor equipped with egg beaters. WO 00/52087 discloses foaming a polymerizable aqueous mixture by first forming an inert gas in the polymerizable, aqueous mixture and then decompressing it to atmospheric pressure. WO 99/44648 discloses neutralizing the monoethylenically unsaturated monomers used to produce a foam with alkanolamines.
It is possible to produce a polymer foam with a defined chemical composition by these processes. However, there is a need to improve the precise control of the morphology of foams.
It is already known that microfluidic processes can be used to produce microparticles. Microfluidic processes are already described in terms of principle in G. M. Whiteside, The Origins and the Future of Microfluidics, Nature 442, 368-372 (2006); M. Hashimoto, P. Garstecki, and G. M. Whitesides, Synthesis of Composite Emulsions and Complex Foams with the use of Microfluidic Flow-Focusing Devices, small 3 (10), 1792-1802 (2007); J. D. Tice, H. Song, A. D. Lyon, and R. F. Ismagilov, Formation of Droplets and Mixing in Multiphase Microfluidics at Low Values of the Reynolds and the Capillary Numbers, Langmuir 19, 9127-9133 (2003); A. M. Ganan-Calvo and J. M. Gordillo, Perfectly Monodisperse Microbubbling by Capillary Flow Focusing, Phys. Rev. Lett. 87 (27), 274501-1-274051-4 (2001); S. L. Anna, N. Bontoux, and H. A. Stone, Formation of dispersions using “flow focusing” in microchannels, Appl. Phys. Lett. 82 (3), 364-366 (2003).
Microfluidic processes and the microscale process technology components used here are notable for the following characteristics: small characteristic lengths in the submillimeter range (a few to a few hundred micrometers), for example from 10 to 1000 micrometers, especially from 100 to 750 micrometers, with small volumes (in the range from 1 nanoliter to 1 femtoliter) of the resulting bubbles, which cause a high surface-to-volume ratio of preferably at least 1000 m2/m3, and also extremely small Reynolds numbers of less than 1000, especially between 1 and 1000, preferably between 1 and 250, especially between 1 and 100. This has the consequence of purely laminar flows, such that the mixing of chemical solutions is limited to purely diffuse operations (instead of kinetic operations). Residence times of substances in microfluidic equipment are generally very short (fractions of seconds), but can be adjusted as desired to the intended reaction.
It is already known that monodisperse particles of defined size and morphology can be produced by microfluidic processes; see, for example, W. Jeong, J. Kim, S. Kim, S. Lee, G. Mensing, and D. J. Beebe, Hydrodynamic microfabrication via “on the fly” photopolymerization of microscale fibers and tubes, Lab Chip 4, 576-580 (2004); V. Hessel, C. Serra, H. Löwe and G. Hadziioannou, Polymerisationen in mikrostrukturierten Reaktoren: Ein Überblick, Chem. Ing. Tech. 77 (11), 1693-1714 (2005); S. Xu, Z. Nie, M. Seo, P. Lewis, E. Kumacheva, H. A. Stone, P. Garstecki, D. B. Weibel, I. Gitlin, and G. M. Whitesides, Generation of Monodisperse Particles by Using Microfluidics: Control over Size, Shape, and Composition, Angew. Chem. 117, 734-738 (2005); Z. Nie, S. Xu, M. Seo, P. C. Lewis, and E. Kumacheva, Polymer Particles with Various Shapes and Morphologies Produced in Continuous Microfluidic Reactors, J. Am. Chem. Soc. 127, 8058-8063 (2005); M. Seo, Z. Nie, S. Xu, M. Mok, P. C. Lewis, R. Graham, and E. Kumacheva, Continuous Microfluidic Reactors for Polymer Particles, Langmuir 21, 11614-11622 (2005); S. Abraham, E. H. Jeong, T. Arakawa, S. Shoji, K. C. Kim, I. Kim, and J. S. Go, Microfluidics assisted synthesis of well-defined spherical polymeric microcapsules and their utilization as potential encapsulants, Lab Chip 6, 752-756 (2006); H. Zhang, E. Tumarkin, R. Peerani, Z. Nie, R. M. A. Sullan, G. C. Walker, and E. Kumacheva, Microfluidic Production of Biopolymer Microcapsules with Controlled Morphology, J. Am. Chem. Soc. 128, 12205-12210 (2006); J. L. Steinbacher et al., Rapid Self-Assembly of Core-Shell Organosilicon Microcapsules within a Microfluidic Device, J. Am. Chem. Soc. 128, 9442-9447 (2006); J.-W. Kim, A. S. Utada, A. Fernandez-Nieves, Z. Hu, and D. A. Weitz, Fabrication of Monodisperse Gel Shells and Functional Microgels in Microfluidic Devices, Angew. Chem. 119, 1851-1854 (2007); C. SERRA, N. Berton, M. Bouquey, L. Prat, and G. Hadziioannou, A Predictive Approach of the Influence of the Operating Parameters on the Size of Polymer Particles Synthesized in a Simplified Microfluidic System, Langmuir 23, 7745-7750 (2007).
US 2007/0054119 A1 already discloses the use of microfluidic systems and techniques to produce monodisperse particles which comprise metal and/or nylon, a polymer precursor being conducted into a microfluidic channel to form the particles and being hardened in the channel.
WO 2005/103106 A1 also discloses producing polymer particles of defined shape and morphology by injecting different fluids into a microfluidic channel and hardening.